1. Field of the Invention
The present invention relates to mechanisms which enable to adjust the length of a tool shank and more particularly to an improved mechanism capable of adjusting length of the shank of a tool (e.g., screwdriver) in a sectionless manner.
2. Description of Related Art
Screwdrivers having a length adjustable shank are well known. Typically, the adjustment is limited to a number of fixed positions. That is, the length of screwdriver can only be adjusted to one of a plurality of predetermined ones. Such adjustment is not satisfactory. Thus, a prior mechanism capable of adjusting length of a screwdriver in a sectionless manner is available as shown in FIGS. 1 to 3. A sleeve 2 is forwardly extended a short distance from a handle 1. The sleeve 2 has a bore of hexagonal section. A length adjustment mechanism is provided in the handle 1 and comprises a shell 3 having a plurality of balls 4 equally spaced around its outer surface and an enlarged aperture plate in its front end, a cylinder 5 having a bore with a front flared end 6 such that the balls 4 are adapted to urge against the flared end 6 of the bore when the cylinder 5 is put on the shell 3, a coil spring 7 compressed between a rear end of the shell 3 and an internal shoulder of the handle 1, a cylindrical member 8 having outer threads adapted to threadedly secure to inner threads of the handle 1 with the spring 7 disposed therein, a first ring 9 having a plurality of grooves 10 put on the shell 3, front and rear second rings 11 in which the front second ring 11 is sandwiched between the first ring 9 and the aperture plate of the shell 3, the rear second ring 11 is sandwiched between the first ring 9 and a front end of the cylinder 5, and each second ring 11 has a plurality of peripheral slopes 12 matingly engaged with the grooves 10, and a spring biased sliding button 14 having a knurled surface provided in a slot 13 on an outer surface of the handle 1, the button 14 has its bottom fastened by the first ring 9. A shank 15 inserted through the sleeve 2 and the shell 3 has its rear end slidably fastened in the handle 1. In an inoperative state, the spring 7 is expanded to position the above components in which the shank 15 is fastened by the inwardly urged balls 4.
For adjusting length of the shank 15 (see FIGS. 2 and 3), slide the button 14 from one position (e.g., one end of the slot 13) to the other position (e.g., the other end of the slot 13). As such, the first ring 9 is actuated to push the second rings 11 rearward and forward respectively as a result of a coaction of the grooves 10 and the slopes 12. And in turn, the shell 3 moves forward to further compress the spring 7 and thus move the balls 4 toward the mouth of the flared end 6 (i.e., having a larger diameter). The shank 15 is disengaged from the balls 4 and is thus free to slide. That is, a user can adjust the length of the shank 15 at this operative state. Alternatively, rather than sliding the button 14 the user may exert a great force to pull the shank 15 outward or push the shank 15 inward directly for carrying out the above adjustment.
However, the prior mechanism suffered from the following disadvantages including being relatively complex in construction, costly to manufacture, trouble-prone, unreliable in use, much force being exerted in the length adjusting operation, and compromised shank fastening after a short period of use time. Thus, the need for improvement still exists.